Dielectric ceramic compositions have found use in the field of electronic communications in such components as filters and resonators. In recent years, the range of frequencies used in electronic communications has expanded so that higher frequencies, i.e., those in the microwave range, are increasingly utilized. A filter may be employed to select a signal within a specific frequency range. The frequency range selected by the filter is referred to as the resonant frequency. Such filters may be based upon a block of dielectric material, often a ceramic material. The resonant frequency of the filter is determined by the dielectric properties of that material and by the dimensions of the block. In general, a dielectric material is required which has a low dielectric loss (indicated by a low dielectric loss factor) in order to minimize energy absorption by the dielectric material that would otherwise reduce resonant signal intensity. The Q factor is defined as the inverse of the dielectric loss factor. Therefore, a relatively lower loss factor results in a relatively higher Q factor. In general, a higher dielectric constant allows the design of a filter with reduced dimensions. For resonant frequencies above about 2 GHz, however, it becomes more difficult to obtain a functional filter because of the small dimensions necessitated by the shorter wavelengths. Thus, a material with a lower dielectric constant, and lower dielectric loss factor (high Q factor), is needed in order to maintain the dimensions of the filter in a range conducive to manufacturing limitations. A percentage fired density approaching 100 also is conducive to achieving a high Q factor.
Conventional dielectric ceramic materials made of alumina or modified alumina do not exhibit sufficiently high Q factor values along with sufficiently low temperature coefficients for satisfactory use as filters and resonators in the microwave frequency band. Additionally, these conventional materials are limited in that they require sintering at relatively high peak soak temperatures of about 1550.degree. C. The peak soak temperature is the maximum (peak) temperature achieved during sintering; it is at this temperature that the material remains (soaks) for a period of time.
Furthermore, under normal operating conditions, a filter is typically subjected to a range of temperatures. As temperature changes, the filter's dimensions are altered by thermal expansion or contraction of the filter material. This results in a shift in resonant frequency. Dielectric properties are affected by a change in temperature, also tending to shift the resonant frequency. The change in resonant frequency caused by a change in temperature is termed the temperature dependence of the resonant frequency. The temperature coefficient (T.sub.f) expresses the frequency shift caused by a change in temperature of 1.degree. C. For example, a T.sub.f of +5 means that the resonant frequency shifts upward by five ppm with a temperature change of 1.degree. C. A T.sub.f of -5 means that the resonant frequency shifts downward by five ppm with a temperature change of 1.degree. C. A temperature coefficient approaching zero (0) is preferred to minimize the shift in resonant frequency due to variations in operating temperature.